The present invention relates to vitreous glass beads or microspheres as used for enabling retro-reflection on road markers, airport runways, or signs and the like.
Most highway guidance lines, such as center lines, edge lines and lane markers, depend upon some sort of light-reflecting device for making them more visible at night when the only source of illumination is the light from motor vehicle head lamps. Such reflecting devices can be cube corners, glass microspheres, or simply light colored objects protruding above the pavement surface.
A plain white line painted on the surface or even a plain white plastic line adhered to the road surface would be barely visible even at a distance as near as 100 feet because of the extremely shallow angle of the light emanating from head lamps and impinging upon the road surface. Most of the incident light is scattered and thus reflected away from the vehicle and very little returns by reflection for the operator to detect. Use of light-reflecting devices such as the above-mentioned, incorporated with the painted or other light-colored line, can increase a motorists detection of the line out to many hundreds of feet. For example, the incorporation of transparent glass microspheres, ranging in size from a few thousandths of an inch in diameter to as much as a tenth of a inch, produce a better light reflection through an effect in which the microspheres serve as miniature optical lenses which focus the incident light from the head lamps into a tiny spot located a slight distance behind the rear surface of the microspheres. The focused spot of light falling upon a pigmented material after undergoing scattering is then partially reflected back upon itself and reaches the motorist's eyes by a phenomenon called retro-reflection. Because of light scattering by the pigmented binder in which the microspheres are partially embedded, only a small percentage of the incident light is returned by retro-reflection; but even this is considerably more light than is the case of an ordinary painted line. During daylight, the ordinary painted line is easily seen by a motorist for thousands of feet because of the abundance of ambient overhead skylight incident upon the line.
The principle of using glass microspheres as light-reflecting lenses for highway markers was first disclosed in U.S. Pat. No. 2,043,414 in 1936. Soda-lime-silicate glass, such as window glass with a refractive index of 1.5, is commonly used as the medium for the microspheres because it is relatively inactive chemically and is a very hard material. This glass, forming the microspheres, generally causes the incident light to come to a focus some distance behind the rear surface of the microsphere. An increase in the brightness can result, however, when the light comes to a focus upon the rear surface of the microsphere itself. This occurs when a glass with a higher index of refraction is used. The distance behind the rear surface of a glass microsphere where the incident light comes to a focus is a function of the refractive index of the glass. As the refractive index increases from a value of approximately 1.5, the focus point moves in closer to the rear surface of the microsphere, reaching this surface when a refractive index value of approximately 1.9 is attained. At this point, the majority of the incident light is returned back upon itself in a retro-reflected beam.
If the rear surface is covered with a highly specular light-reflecting metal such as aluminum, chromium, silver or some other specularly reflective material, then all of the incident light beam is returned except for small losses due to absorption and other minor effects, such as spherical aberration. Even without such a reflective coating, however, the returned light beam is considerably brighter than it would be with a lower refractive index glass because the scattered light in the focused spot is very near the rear surface of the sphere and thus most of it re-enters the sphere and produces a brilliant retro-reflected beam.
Unfortunately, however, 1.9 refractive index glass microspheres are generally not satisfactory for use with highway marking lines for several reasons, among which are lack of resistance to attack from atmospheric conditions, poor resistance to crushing from motor vehicle tire impacts, devitrification, color impurities, high cost, and other defects.
Resort has been attempted to harder metal-oxide ceramic "glass" or optical microspheres made by a sol-gel process disclosed in U.S. Pat. No. 4,564,556 and in which the refractive index is in the range of 1.75 to 1.76, providing a surface hardness greater than soda-lime-silicate glass. U.S. Pat. No. 4,349,456 also discloses transparent or clear non-vitreous ceramic microcapsules made by a sol-gel process using metal oxides which are hollow spheres capable of being subsequently liquid filled for the different purpose of filler material in plastic compounds. Such ceramic "glass" is also impervious to noxious atmospheric fumes such as the sulfurous emission which forms sulfuric acid in motor vehicle exhaust vapors. Although these microspheres have a higher index of refraction than the commonly used vitreous silicate glass, they are still short of the optimum brightness where the incident light focuses at the rear surface of the microsphere. According to the '556 patent, it should be possible to produce a ceramic microsphere with a refractive index of about 1.9; but there are evidently practical problems with this particular sol-gel system which so far have precluded its use for this purpose.
For the first time, in accordance with the present invention, it has been discovered how to produce the novel result of an optical microsphere with the surface hardness and density of a ceramic microsphere, but with the clarity and optimal retro-reflective brightness equivalent to a vitreous glass microsphere of approximarely 1.9 refractive index, and which causes focusing at or near the rear surface of the microsphere for efficient retro-reflection.
This is achieved through a novel encapsulation of a vitreous glass microsphere with a hard, clear non-vitreous metal oxide ceramic shell.